Jeff Mitton: Pandora pinemoth populations erupt to defoliate pines

A Pandora pinemoth waits, having expended all of its energy. (Jeff Mitton / For the Camera)

Jeff Mitton Natural Selections

I had stopped at the little town of Big Water, Utah, just west of Lake Powell, to fill up on gas for the trip over Smoky Mountain to Escalante.

I was amazed to see a dense population of large purple and grey moths at the gas station. The attendant told me that the moths had appeared in the middle of the night, attracted to the bright lights high over the gas pumps.

The large lampposts, 30 feet tall and a foot in diameter, had been completely covered with moths. Some of the moths had left as the day brightened, but several hundred remained, moribund.

These were Pandora pinemoths, Coloradia pandora, and they would fly no more. Males die after mating and females die after depositing eggs, for adults have no mouthparts and cannot feed.

Two years are needed for the pinemoth to complete its life cycle.

Females deposit their eggs in July and August, and eggs take several weeks to hatch. Young larvae begin to feed in late summer, eating the pine needles of ponderosa and lodgepole pines.

As cold temperatures slow their metabolic rates, they gather at the bases of needle clusters for the winter. They continue feeding in spring and early summer, but in July they crawl out of the trees to burrow into the soil, where they develop into pupae. Pupae spend the second winter in the soil, and they emerged as adults in the following June and July.

Males fly about to find females, who do not fly until they have mated.

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Pandora pinemoths are among the largest of our forest insects. Larvae start off ¼-inch long, but grow to be almost 3 inches long. Pupae are 0.6 inches thick and 1.5 inches long. Adults grow to have body lengths of 1 to 1.5 inches and wingspans of 3 to 4.5 inches.

My encounter at Big Water was the first and only time that I have seen pinemoths — Toby Hammer, a Ph.D. student in ecology and evolutionary biology, identified them for me.

They are native to western North America, but are limited to the Sierra Nevada in California and adjacent mountains of southern Oregon, the Uinta Range in Utah, and the Rocky Mountains in north central Colorado and southern Wyoming, with an small isolated population in the San Francisco Peaks near Flagstaff.

Big Water is far from any of the known pinemoth areas.

I learned that Pandora moths erupt to create epidemics of defoliation principally in ponderosa and lodgepole pines, though occasionally they feed on Jeffrey and Coulter pine.

Defoliation decreases growth rates, but does not usually kill trees, for the larvae do not harm needle buds. Epidemics typically last 5 to 8 years, and this prolonged herbivory leaves a permanent signature in the radial growth that can be measured from tree cores.

A study of tree growth in 14 mature ponderosa pine forests in south central Oregon detected 22 outbreaks in the last 622 years. Intervals between outbreaks were highly variable, from nine to 156 years. The last epidemic in Colorado was in lodgepole pine between 1937 and 1940, but I don't remember that.

I saw a dense population of Pandora pinemoths in Big Water, but I think that the closest stands of either ponderosa or lodgepole are more than 50 miles away, probably much further than pinemoths fly.

Furthermore, I found one account in the literature that indicated that some pinemoths fed on pinyon pine — the local hills have a sparse pinyon-juniper woodland. That account also suggested that the moths feeding on pinyon are perhaps a different, undescribed species.

The possibility of an eruption of an undescribed species of giant pinemoths gives me a reason to return to the pinyon-juniper woodland west of Lake Powell, near the quiet town of Big Water.

Jeff Mitton, mitton@colorado.edu, is a professor in the Department of Ecology and Evolutionary Biology at the University of Colorado.

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